5-Fluorouracil (hereinafter abbreviated to as 5-FU) has been used clinically in treatment of malignant tumors, for example, carcinoma, sarcoma, skin cancer, cancer of the digestive organs, cancer of the breast. 5-FU, however, shows serious adverse reactions such as nausea, alopecia, diarrhea, stomatitis, leukocytic thrombocytopenia, anorexia, pigmentation, edema, general fatigue, etc., in application over a long period of days, and it is difficult to administer orally because of its strong toxicity. Recently, the occurrence of and death rate owing to malignant tumors show a tendency to increase, and it is desired to develop antitumor agents for which the activity is strong but which are less toxic, and which can be orally administered. In order to achieve the above purpose, chemical modification of a powerful antitumor agent, 5-FU which unfortunately has strong toxicity has been attempted extensively. Representatives of the modified 5-FU derivatives already known are 1-(2-tetrahydrofuryl)-5-fluorouracil [trademark: Ftorafur (Taiho Yakuhin); Japanese Unexamined Patent Publication Nos. 50-50383, 50-50384, 50-64281, 51-146482 and 53-84981], and 1-carbamoyl-5-fluorouracil [ Mitsui Seiyaku; Japanese Unexamined Patent Publication No. 50-148365].